Biomass feedstock may be solely lignocellulosic material or a mixture of lignocellulosic and other materials. Polysaccharide biomass is typically a mixture of starch and lignocellulosic materials. The starch may be contained in grains or a refined starch added as feedstock to form the biomass. The biomass feedstock may also include polymers and other materials.
Enzymes are mixed with the biomass to promote hydrolysis. Mixing ensures the enzymes continually and repeatedly move into contact with chemical reaction sites in the biomass to promote hydrolysis or other degradation of the biomass. In addition, or in place of enzymes, other cellulose degrading organisms and biocatalysts may be added to the biomass to promote hydrolysis or other degradation of the biomass.
The feedstock of lignocellulosic material and enzymes or other degrading materials are mixed together to form the biomass mixture. This biomass mixture may have characteristics similar to a high matter content powder. Liquid may also be added to the biomass mixture to form a high viscosity liquid slurry. Liquid may be added to liquefy the biomass solids and generate a uniform biomass emulsion formed of feedstock and liquids, which have significant differences in their characteristics.
Mixers, constant-stir reactors, and other similar mixing or agitation devices may be used to mix and liquefy the feedstock and enzymes to form the biomass mixture. These devices conventionally are cylindrical vessels arranged vertically and have mechanical mixing devices, such as stirrers having radial arms and blades. These mixing devices generally rotate about a vertical shaft and move through the biomass, with mixing occurring for a period of time depending of the feedstock used.
Enzymatic liquefaction of lignocellulosic feedstock to biomass may require several hours of mixing. A result of the mixing is the reduction of the viscosity of the biomass. The enzymes convert the generally solid biomass composition into liquefied slurry. Biomass pretreated for enzymatic conversion to monomeric sugars typically starts the mixing process having a fibrous or mud-like consistency. The enzymes added to the biomass typically have a relatively low concentration with respect to the biomass. The biomass and enzyme mixture tends to be highly viscous as it enters the mixer and pretreatment hydrolysis reactor system. There could be one or more hydrolysis reactor vessels in the system.
Because of the high viscosity of the biomass entering the hydrolysis reactor vessel, a large force (torque) is needed to turn the mixing devices and properly mix the enzymes with the biomass. The mixing force traditionally limits the size of the mixing vessels. Many of the conventional vessels where mixing occurs tend to be small diameter vessels as the torque needed to rotate the mixing arms increases exponentially with the radial length of the arms. Due to the high viscosity of the biomass, the radial length of the arms is traditionally short so they can move through the biomass. Motors used to turn the mixing arms have a maximum power limitation, contributing to the constraint of the maximum length of the mixing arms. As a result of the constraints of the motor and mechanical strength of the mixing components, the vessels used for mixing the high viscosity pretreated biomass have conventionally been small and narrow.
For these reasons, and others, the mixing vessels for enzymatic liquefaction of lignocellulosic biomass have conventionally been operated in batch rather than continuous mode and frequently require the simultaneous operation of multiple batch mixing vessels to feed a larger downstream vessel.
A large continuous mode mixing vessel capable of mixing the highly viscous biomass and enzymes has recently been developed as described in US Patent Application Publication 2012-125549 (the “'549 Application”). In this system the enzymatic hydrolysis and mixing process relies on physical forces, such as gravity and centrifugal force, to ensure the biomasses are subjected to the desired mechanical forces.
In the continuous mixing and reactor device, a first internal mixing chamber has a cross-sectional area expanding from the biomass inlet to the internal area of a second chamber with a uniform internal cross-sectional area throughout the second chamber. In this system, the biomass reactor contains the rotating mixing device and is coaxial with the reactor vessel. This mixing chamber can be comprised of multiple zones at different elevations in the vessel. The mixing is caused by horizontal paddles or trays and also allows for the movement of the material vertically down the vessel. The liquefied slurry flows from the lower zones of the mixing vessel, with a portion of the slurry pumped or circulated to the upper zones of the vessel to adjust the slowly changing viscosity of the feedstock at the upper elevations of the vessel.
While the use of the system and method described in the '549 Application has allowed for continuous operation of a mixing and reactor vessel, the vertical mixing which results from the method of the '549 Application has been found to reduce the desirable plug flow needed for good control of viscosity reduction as the material moves through the reactor vessel. A “plug flow” refers to a flow with a substantially constant velocity across a given area. The desired plug flow promotes consistent retention time in the reactor vessel and avoids regions in the vessel of stagnant biomass.